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Effect of non-thermal plasma on the bond strength of orthodontic brackets to enamel : an invitro study Arora, Deepak 2015

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EFFECT OF NON-THERMAL PLASMA ON THE BOND STRENGTH OF ORTHODONTIC BRACKETS TO ENAMEL: AN IN VITRO STUDY by  Deepak Arora  M.D.S., The University of Nagpur, 1999 B.D.S., The University of Nagpur, 1995  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  MASTER OF SCIENCE  in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Craniofacial Science)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)   August 2015  © Deepak Arora, 2015 ii Abstract  Objectives: Non-thermal plasma (NTP) has been used to modify enamel and dentin surfaces and improve the interfacial bonding of dental composite restorations. NTP is shown to cause  super hydrophilic surface by decreasing the contact angle measurements and improve the quality of the adhesive - enamel and adhesive - dentin interface. We sought to determine the effects of NTP treatment on the bond strength of brackets to enamel. We investigated the application of NTP alone and in combination with phosphoric acid (PA) etching and assessed the outcomes after 24 hours and 1 month. Methods: 84 extracted pre-molars washed and disinfected were divided into 2 broad groups: No-treatment and Treatment group. No - treatment group consisted of 12 premolars on which orthodontic bracket bonding was performed without any surface - treatment except for polishing with pumice. Treatment group consisted of 72 premolars which were divided randomly into 3 main groups of 24 premolars each. These groups were Group 1: PA Etch (30s), Group 2: PA Etch (30s) + NTP (30s) and Group 3 NTP (30s). The bonded teeth were stored in water at 37° C and tested in shear mode after 24 hours and 1 month (n= 12). The fracture mode and adhesive remnant index were determined on de-bonded surfaces. SEM pictures were taken from enamel surfaces after each treatment.    iii Results: During the first 24 hours of testing, SBS was maximum with Etch+ NTP and Etch treated group followed by NTP group ( P < 0.05). After 1 month of ageing the SBS was maximum with the Etch group followed by Etch+ NTP and NTP group ( P < 0.05). ARI scoring was higher with Etch and Etch+ NTP group. SEM pictures showed Type 1 acid etch pattern with Etch and Etch+ NTP group whereas NTP treated group showed no surface changes. Conclusions: NTP application by itself has a potential to bond orthodontic brackets however a longer ageing time will explain more about this feasibility.   iv Preface  This research project was designed by Dr. Deepak Arora under supervision of Dr. Edwin H. K. Yen and the guidance of Drs. Ricardo M Carvalho and N. Dorin Ruse. The data was collected and analyzed by Dr. Deepak Arora who prepared the manuscript with the advice from Drs. N.D Ruse, R M Carvalho and Edwin H.K Yen. The orthodontic brackets tested were donated by Cerum Ortho Organizers Aurum Group.The study was approved by the University of British Columbia office of Research Services Human Research Ethics Board (Certificate Number B13-0142).   v Table of Contents  Abstract .......................................................................................................................................... ii Preface ........................................................................................................................................... iv Table of Contents ...........................................................................................................................v List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Images .................................................................................................................................x List of Abbreviations .................................................................................................................. xii Acknowledgements .................................................................................................................... xiii Dedication ................................................................................................................................... xiv Chapter 1: Introduction ................................................................................................................1  Disadvantages of orthodontic bonding using acid etching ............................................. 2 1.1.1 Bond failure ................................................................................................................ 2 1.1.2 White spot lesions and acid etching ............................................................................ 4 1.1.3 Enamel loss after acid etching and bonding ............................................................... 6 1.1.4 Technique sensitivity .................................................................................................. 7  Some important considerations during orthodontic bonding .......................................... 8 1.2.1 Properties of ideal orthodontic adhesive ..................................................................... 8 1.2.2 What is optimal bond strength .................................................................................... 9 1.2.3 Adhesive remnant index : some interesting facts ....................................................... 9  Review of adhesion science .......................................................................................... 10 1.3.1 Requirements for creating good adhesion................................................................. 11 vi 1.3.1.1 Clean surfaces ................................................................................................... 12 1.3.1.2 Surface roughness ............................................................................................. 12 1.3.1.3 Proper contact angle and good wetting ............................................................. 13 1.3.1.4 Low viscosity adhesives and adequate flow ..................................................... 14 1.3.1.5 Adhesive solidification ..................................................................................... 14  Alternatives to acid etching .......................................................................................... 14 1.4.1 Air-abrasion .............................................................................................................. 14 1.4.2 Laser etching ............................................................................................................. 15  Non-thermal plasma: as an alternative pre-conditioning procedure ............................. 16  Study rationale .............................................................................................................. 17 Chapter 2: Materials and methods .............................................................................................20  Materials used in the study ........................................................................................... 20 2.1.1 Bonding system ......................................................................................................... 20 2.1.2 Non-thermal plasma (NTP)....................................................................................... 20  Experimental method .................................................................................................... 26 2.2.1 Teeth collection ......................................................................................................... 26 2.2.2 Groups ....................................................................................................................... 27 2.2.3 Bracket bonding procedure ....................................................................................... 29 2.2.4 Specimen mounting .................................................................................................. 30 2.2.5 Testing procedure...................................................................................................... 35 2.2.6 Adhesive remnant index ........................................................................................... 38 2.2.7 Scanning electron microscopy .................................................................................. 39 2.2.8 Statistical analysis ..................................................................................................... 40 vii Chapter 3: Results........................................................................................................................41  SBS analysis.................................................................................................................. 41 3.1.1 Comparisons at the 24hour follow-up....................................................................... 43 3.1.2 Comparisons at the 1 month follow-up ..................................................................... 45 3.1.3 Two-way ANOVA (surface treatment and ageing time interaction) ........................ 46  Adhesive remnant index analysis.................................................................................. 49  SEM evaluation ............................................................................................................. 51 3.3.1 SEM evaluation of enamel surfaces .......................................................................... 51 3.3.2 SEM evaluation (adhesive remnant index) ............................................................... 64 Chapter 4: Discussion ..................................................................................................................73 Chapter 5: Conclusions ...............................................................................................................84 References .....................................................................................................................................85    viii List of Tables  Table 2.1 Table of product chemical compositions ...................................................................... 25 Table 3.1 Descriptive statistics for SBS (MPa). ........................................................................... 41 Table 3.2 One-way analysis of variance (ANOVA) on SBS at 24 hours ..................................... 44 Table 3.3 Pairwise multiple comparison procedures (Tukey Test) .............................................. 44 Table 3.4 One-way analysis of variance (ANOVA) on SBS at 1 month ...................................... 45 Table 3.5 Pairwise multiple comparison procedures (Tukey Test) .............................................. 46 Table 3.6 Customized two-way analysis of variance (ANOVA) on SBS x Time ........................ 47 Table 3.7 Pairwise multiple comparison (Tukey Test) ................................................................. 47 Table 3.8 Frequency distribution of the (ARI) , of experimental groups-24 hours ...................... 49 Table 3.9 Frequency distribution of the (ARI) of experimental groups-1 month ......................... 50   ix List of Figures  Figure 1.1 Contact angles indicating good wettability (left) and poor wettability (right ) ........... 13 Figure 2.1 Control and treatment groups ...................................................................................... 28 Figure 3.1 Box-and-whisker charts of SBS at 24 hours and 1 month........................................... 42 Figure 3.2 Line chart of decline SBS from 24 hours to 1 month .................................................. 48 x List of Images  Image 2.1 Transbond™ XT adhesive (3M Unitek, Monrovia, California) .................................. 21 Image 2.2 Orthosolo® primer ....................................................................................................... 21 Image 2.3 Ultraetch™ etching gel ................................................................................................ 22 Image 2.4 Orthodontic brackets: Elite® mini twin® (0.22 slot) Ortho Organizers Inc. .............. 22 Image 2.5 Maxillary premolar bracket. Elite® mini twin® (0.22 slot) Cerum Ortho .................. 23 Image 2.6 Light cure unit: Blue phase style, Ivoclar-Vivadent .................................................... 23 Image 2.7 Non thermal plasma (KinPen™ 09, INP Greifswald, Germany) ................................ 24 Image 2.8 Ney’s dental surveyor .................................................................................................. 32 Image 2.9 (Guide for standardization of test specimen) -modified Ney’s surveyor ..................... 33 Image 2.10 Lowering premolar in the approximate center of the PVC mold ............................... 33 Image 2.11 Pouring acrylic after the premolar with bonded bracket is lowered in PVC block ... 34 Image 2.12 Final acrylic set .......................................................................................................... 34 Image 2.13 Adjustable vice (Odeme company, Brazil) ................................................................ 36 Image 2.14 Chisel edge (Odeme company, Brazil) ...................................................................... 36 Image 2.15 Orientation of blade to enamel- bracket interface ...................................................... 37 Image 2.16 Shimadzu AGS-X series table-top type precision universal testing machine ............ 37 Image 2.17 Carl Zeiss Jena microscope, Germany ....................................................................... 38 Image 3.1 SEM (500x) photo of (Nt-control) enamel surface ...................................................... 52 Image 3.2 SEM (1500x) photo of (Nt-control) enamel surface .................................................... 53 Image 3.3 SEM (5000x) photo of (Nt-control) enamel surface .................................................... 54 Image 3.4 SEM (500x) photo of enamel surface exposed to acid etching (30 s). ........................ 55 xi Image 3.5 SEM (1500x) photo of enamel surface exposed to acid etching (30 s). ...................... 56 Image 3.6 SEM (5000x) photo of enamel surface exposed to acid etching (30 s). ...................... 57 Image 3.7  SEM (500x) photo of enamel surface exposed to acid etching (30 s) + NTP (30 s) .. 58 Image 3.8 SEM (1500x).photo of enamel surface exposed to acid etching (30 s) + NTP (30 s) . 59 Image 3.9 SEM (5000x) photo of enamel surface exposed to acid etching (30 s) +NTP (30 s) .. 60 Image 3.10 SEM (500x) photo of enamel surface exposed to NTP (30 s) . ................................. 61 Image 3.11 SEM (1500x) photo of enamel surface exposed to NTP for (30 s) ........................... 62 Image 3.12 SEM (5000x) photo of enamel surface exposed to NTP (30 s) ................................. 63 Image 3.13 SEM (30x) photo of a representative sample from no-treatment (control) group (24 hours) ............................................................................................................................................ 66 Image 3.14 SEM (30x) photo of a representative sample from Etch (24 hours) group ................ 67 Image 3.15 SEM (30x) photo of a representative sample from Etch +NTP (24 hours) group ..... 68 Image 3.16 SEM (30x) photo of a representative sample from NTP (24 hours) group ............... 69 Image 3.17 SEM (30x) photo of a representative sample from Etch (1 month) group ................ 70 Image 3.18 SEM (30x) photo of a representative sample from Etch+ NTP (1 month) group...... 71 Image 3.19 SEM (30x) photo of a representative sample from NTP (1 month) group ................ 72  xii List of Abbreviations  NTP                    Non thermal plasma E                         Etching ARI                     Adhesive remnant index SEM                    Scanning electron microscope GSTS                  Guide for standardization of test specimen WSL                   White spot lesions SBS                     Shear bond strength xiii Acknowledgements  I would like to thank my supervisor Dr. Edwin. H.K. Yen for guiding me for the past 3 years in the completion of my Masters thesis. Your encouragement and insight at every step of the way is greatly appreciated. My sincere thanks to Dr. Ricardo M Carvalho for being supportive with his innovative ideas and keeping my spirits high as I went by to complete the project. I want to thank you for giving me this wonderful learning opportunity. Also I want to express my deep gratitude to Dr. N. D. Ruse for helping me design and execute this project. Your constructive criticism and feedback from time to time in drafting the manuscript has really helped me to complete this project. I want to thank Cerum Ortho Organizers (Aurum Group) for donating the orthodontic brackets for this study. Finally, I want to thank my wife Puja for her unconditional support and my son Siddharth and daughter Saachi for being there for me whenever I needed them and making me believe in myself.  xiv Dedication For my parents and my brother….. Thanks for all your encouragement and support for which I will always be indebted to you.  1 Chapter 1: Introduction  Early orthodontic systems before the advent of bonding brackets to enamel involved banding every tooth in the mouth. First, separators were placed to create spaces between teeth. Then, each individual band was fit and adapted to the contours of the tooth. Finally, the bands were cemented into place and excess cementing material was removed. With a proper fitting band, three dimensional control of the surrounded tooth was possible via welded brackets through which a wire was ligated. What once was considered a common practice, banding the entire mouth is now regarded as a tedious process and is marred with many disadvantages like extensive chair-side time, more pronounced effect on periodontal health, need for frequent screening for decalcification of underlying tooth structure, additional arch space requirement to accommodate the width of each band and need to separate all teeth prior to band fitting which may cause discomfort to patients (Brantley et al., 2000).  Michael G Buonocore (Buonocore, 1955) revolutionized dentistry with his historical  research of phosphoric acid etching and bonding of acrylic to enamel. Subsequently, Bowen (Bowen, 1956) launched his research and synthesized the bisphenyl A glycidyl di-methacrylate (Bis-GMA) resin which led to the first successful production of a composite resin for filling teeth. There were many hurdles in the development of a good adhesive system for bonding orthodontic brackets in the initial days considering high expectations from them including ability to withstand stresses of mastication , remain in place for reasonable treatment period and also be easily removed at the conclusion of treatment (Newman et al., 1968).  2 Significant improvements to adhesives, bracket bases, and bonding technique have answered most of these demands, including the ability to bond to non-enamel surfaces (Rossouw, 2010), bond failure is still a common problem.   Disadvantages of orthodontic bonding using acid etching  Even a breakthrough such as direct bonding using acid etching did not arrive without its own short-comings. Bond failures (Zachrisson, 1977), decalcification and predisposition to white spot lesions (Chang et al., 1997), inadvertent loss of enamel during bonding and clean up procedures (Van Waes et al., 1997)  technique sensitivity (Sethusa et al., 2009) and retention of resin tags inside enamel that can lead to subsequent potential discoloration of enamel (Gwinnett, 1971; Silverstone, 1977) are still some of the common problems associated with acid etching.  1.1.1 Bond failure  Orthodontic treatment with brackets generally takes 2-3 years (Beckwith et al.) and bracket loss during treatment is the bane of orthodontists, requiring additional in-office time, additional expense to replace the dislodged brackets and in some cases extended total treatment time (Oesterle et al., 2008) leading to patient inconvenience (Finnema et al., 2010b).  Sometimes conventional bonding techniques are insufficient when bonding brackets in uncontrolled humidity or on irregular enamel surfaces, such as deciduous teeth, hypo-calcified enamel, and fluoridated enamel surfaces.   3 Also greater bond strengths may be preferred with non-compliant patients, when diet is unchanged to meet treatment needs, or when destructive chewing habits lead to bond failure.  Reported clinical bracket-failure rates vary from 2.7 % in a short-term study (Ash et al., 1996) to 29 % for mandibular molars (Zachrisson, 1977). Millett and his co-workers in a 5 year clinical review of bond failure with light cure resin adhesive concluded that there was an overall bracket failure rate of 6 % (Millett et al., 1998). In another study (Zachrisson, 2007) the authors concluded that the overall failure rate in terms of loose brackets was 11 %, but, of course, the de bonding varied according to the  different teeth. The maxillary first molars showed the highest de-bonding rates (27 %), whereas more than 90 % of the incisor and canine brackets stayed on throughout treatment. In the mandible, all six anterior teeth had failure rates below 10 %, while the premolars and, particularly, the molars showed higher rates (24 %). Studies have shown that about 2 % to 6 % of indirectly bonded brackets still fail for different reasons (Deahl et al., 2007; Miles et al., 2003; Thiyagarajah et al., 2006).There was no difference between failure rates between direct and indirect bonding (Deahl et al., 2007). Bonding of orthodontic brackets to fluorosed enamel is a challenge for all dental clinicians (Miller, 1995) and Lupan et al (Lupan et al., 2011a) have shown that the rate of bond failure with 37 % phosphoric acid is high in patients with dental fluorosis.  There have been many strategies to decrease bond failure rate so far including use of new adhesive materials (Vicente et al., 2004), innovative bracket base designs (Bishara et al., 1999), enamel etching and conditioning procedures (Canay et al., 2000),introduction of adhesion promoters (Wenger et al., 2008) ,use of different curing lights (Evans et al., 2002) and even 4 advent of lasers which have been employed for curing composite resin (Von Fraunhofer et al., 1993) to enhance bracket bond strength. While some of these have shown some degree of success in improving the bond strength of brackets, no systematic analysis has been employed so far to evaluate their efficacy in reducing bracket bond failure rates.  1.1.2 White spot lesions and acid etching   Despite the fact that the acid etching method is a useful procedure in orthodontics, a potential disadvantage is the possibility of decalcification, which increases the predisposition of enamel to dental caries critically under orthodontic brackets. Enamel decalcification and white spot lesions (WSL) are common problems that occur in 2 % to 96 % of orthodontic patients (Chang et al., 1997). WSL develop in areas associated with orthodontic appliances, such as locations for bands, brackets, arch wires, and wire or elastomeric ligatures. Moreover, most orthodontic patients are adolescents with poor oral hygiene practices which increases the likelihood of plaque accumulation; and in turn leads to demineralization of intact enamel (Gorelick et al., 1982). Acid etching prior to enamel bonding is a possible causative factor in the decalcification associated with orthodontic treatment (Knösel et al., 2012) as etching demineralizes the enamel surface at depths ranging from 3.5 μm to 27.1μm (Legler et al., 1990). Acid-etched enamel can re-mineralize, but the amount of time required for this to occur is variable, and the extent of recovery is incomplete (Garberoglio et al., 1979).The acid-etched surface more importantly allows the less mineralized underlying enamel to be exposed to a potentially acidic microenvironment (Davis, 1986) and etched enamel exposed to cariogenic solutions has been repeatedly shown to be more severely affected than un-etched enamel (Savarino et al., 2002; Steffen, 1996). 5  Previous experiments on bovine enamel, including extensive etching intervals, suggested that white spot lesions might be triggered iatrogenically by surplus etching ( phosphoric acid,50 wt. percent for 1 minute) of enamel areas that are subsequently not covered by bracket bases or bonding material, because etching removes parts of the outer enamel layer and results in a rough, retentive surface. Researchers like Lehman et al (Lehman et al., 1981) and Kuhar et al (Kuhar et al., 1997) have noted that etching and grinding significantly increases the permeability of dental enamel. The results of their study demonstrated that acid-etched and ground dental enamel surfaces are less protected in-vivo and consequently, unless the tooth is properly protected, are more susceptible to carious lesions. This can be especially severe in teeth in which almost the complete facial enamel surface is etched but subsequently incompletely covered by bonding material or sealers.   Gorelick and his group (Gorelick et al., 1982) found that for the teeth studied, there was no difference in white spot formation in those that were banded or bonded. They stated that-“obvious degree of iatrogenic damage during orthodontic treatment suggests the need for preventive programs using fluoride and further clinical research.” Another study concluded that excessive surplus orthodontic etching of the complete labial enamel surface, must be avoided to prevent iatrogenic white spot lesions and noted that etching times not exceeding 15 seconds are favorable (Knösel et al., 2012).  According to Knösel et al (Knösel et al., 2007) WSL are not only disturbing esthetically, especially in the anterior teeth but also are an initial stage of enamel caries, producing outer 6 enamel layers with significantly reduced mineralization (Palamara et al., 1986) that will progress to a stage at which restoration will be needed, if inadequate oral hygiene prevails (Featherstone, 2000). Hess at al in their interesting study have concluded that filled resin sealants protect the teeth from decalcification (Hess et al., 2011).  1.1.3 Enamel loss after acid etching and bonding  Damage to enamel during bonding and de bonding is a clinical concern. According to Finnema et al (Finnema et al., 2010a) minimum enamel damage with the maximum clinically useful bond strength is optimal and reduction in etching time and acid concentration that produce an optimal bond should be strived for. The amount of enamel lost during the removal of  adhesive may be of clinical significance because of the removal of a major part of the protective fluoride-rich layer of enamel (Brown et al., 1978) who also observed that in normal human enamel  there is highest fluoride concentration at the surface, and then there is a rapid decline in concentration in the first 20 µm of enamel . Other studies (Legler et al., 1990) have measured the depth of etch after the use of acid concentrations: 37 %, 15 %, and 5 % phosphoric acid and observed a stepwise decrease in the calculated depths of etch with decreasing acid concentration and duration of etching. The calculated etch depths ranged from 27.1 μm by etching with 37 % phosphoric acid for 60 seconds to 3.5 μm by etching with 5 % phosphoric acid for 15 seconds.  Enamel removal occurs during various processes. Abrasive wear removes approximately  2 μm/year, routine etching 3-10 μm, enhanced mechanical interlocks remove another 25 μm. Thus, a total of approximately 35 μm is removed in the process of bonding and de-bonding; 7 slightly more enamel can be removed depending on the instrument and procedure used for de-bonding (Graber et al., 2011; Rossouw, 2010). Also in another interesting study Sinha et al (Sinha et al., 1995) concluded that reduced adhesive on enamel following de-bonding would require less cleanup and therefore reduced risk of damaging enamel and enamel loss.   1.1.4 Technique sensitivity  Contamination by saliva, blood, or bleaching agents, can cause bonding agents to be technique sensitive which simply means that they may fail prematurely if steps are not followed meticulously (Powers et al., 2010). Özer et al (Özer et al., 2008) concluded that a simplified technique that still produces clinically useful bond strength would be advantageous. Sethusa and his group concluded that orthodontic bracket adhesion involves multistep procedures which are technique sensitive to various factors within the oral environment (Sethusa et al., 2009).  While it is often claimed that the higher the number of application steps, the higher the risk to make manipulation errors (higher technique sensitivity) it is obvious that each step has its specific function and is therefore best carried out separately in order to achieve the most optimal result. Although recent bonding systems are reliable in conservative dentistry, improvements in adhesive resins are still necessary to minimize technique sensitivity as well as to produce more durable materials. According to Bishara et al (Bishara et al., 2002) new materials should require a reduced number of steps and chair time.   8  Some important considerations during orthodontic bonding  1.2.1 Properties of ideal orthodontic adhesive 1. have adequate bond strength while maintaining unblemished enamel after de-bonding orthodontic brackets (Bishara et al., 2002) 2. minimum technique sensitivity (Bishara et al., 2002)  3. be durable in nature (De Munck et al., 2005)  4. provide continuous, prolonged fluoride release, and have adequate bond strength to satisfactorily retain orthodontic brackets (Vorhies et al., 1998) 5. not undergo shrinking during setting (Newman et al., 1968) 6. sufficiently cross linked to minimise water sorption (Retief et al., 1970) 7. adequate wettability and penetration without undue slumping and bracket drift (Brantley et al., 2000) 8. rapid initial set to minimise setting shrinkage (Brantley et al., 2000) 9. overall water absorbing tendency should be minimal (Brantley et al., 2000) 10. fluid adhesive be converted to solid within permissible temperature (oral temperature) and polymerisation may proceed to chemical completion. This is expressed as degree of conversion of reactive groups in the starting monomer to desired solid polymer state (Brantley et al., 2000)    9  1.2.2 What is optimal bond strength  Retief (1974) highlighted the different factors with respect to optimal bond strength. He showed that enamel fractures can occur with bond strengths as low as 13.5 MPa. The minimum clinically adequate total bond strength according to Reynolds (Reynolds, 1975) appears to be between  5.8-7.8 MPa . It was also shown by Bishara et al (Bishara et al., 1993) that a mean safe de-bonding strength should be less than 11.2 MPa. Therefore it seems that the optimum range for bond strength is thus between 5.8 and 13.5 MPa (Rossouw, 2010). Since it has never actually been tested whether 6 to 8 MPa is a sufficient in-vitro bond strength for clinical use, the use of this reference value has been criticized before (Eliades, 2002),(Eliades et al., 2000). The authors noted that extrapolation of absolute values and comparing them with a supposedly ‘‘clinically acceptable’’ reference value should be avoided (Eliades et al., 2005).  1.2.3 Adhesive remnant index : some interesting facts  Årtun and Bergland (Årtun et al., 1984) have used an adhesive remnant index (ARI) to evaluate the amount of adhesive left on the tooth after de bonding. A tooth is scored on a four point scale as follows: score of 0 = no adhesive left on the tooth; score of 1 = less than half of the adhesive left on the tooth; score of 2 = more than half of the adhesive left on the tooth; and score of 3 = all adhesive left on the tooth with a distinct impression of the bracket mesh. This is generally accomplished by observing the amount of adhesive left on the bracket following de bonding, and subtracting it from 100 %. There has been debate whether or not ARI scores reflect a difference in bond strength (Montasser et al., 2009).While some studies demonstrated a correlation or a 10 parallel between shear bond strength and ARI (Parrish et al., 2011) (Mirzakouchaki et al., 2012), others have shown the contrary, suggesting the amount of adhesive remaining following de-bonding is not related to shear bond strength, but is instead governed by numerous factors, including bracket base design and adhesive properties (O'brien et al., 1988).  A failure pattern that results with most of the adhesive remaining on the tooth can be interpreted as protection of the enamel from the stresses of de-bonding and subsequent fracture , with the disadvantage of having more adhesive to remove mechanically after removing the bracket (Bishara et al., 2008) . On the other hand, reduced adhesive on the enamel following de-bonding will require less cleanup, and the risk of damaging the enamel by mechanical resin removal and polishing is reduced (Sinha et al., 1995) . Thus, there has not been a consensus on whether more or less adhesive remaining is preferred or most beneficial.   Review of adhesion science  Adhesion is defined as the molecular attraction exerted between surfaces of bodies in contact or the attraction between molecules at an interface (Dictionary, 2006). Adhesion science focuses on understanding the material properties associated with formation of the interfaces, changes in the interfaces with time, and events associated with failure of the interfaces (Marshall et al., 2010). Bonding involves potential contributions from physical, chemical, and mechanical sources but primarily relies on micro-mechanical interaction for the interfacial strength of the assembly (Marshall et al., 2010). Physical bonding forces are generally very weak. Chemical bonding is stronger but also very difficult to produce in a dense manner across an interface. While micro-mechanical interlocking 11 is believed to be a prerequisite to achieve good bonding within clinical circumstances, the potential benefit of additional chemical interaction between functional monomers and tooth substrate components has recently regained attention (Nagakane et al., 2006). Each dental adhesive contains a specific functional monomer that determines its actual adhesive performance to tooth tissue. 4-methacryloxyethyl trimellitic acid(4-MET) is well-known as one of the functional monomers mostly available and consequently widely used in commercial adhesives. Chemical bonding of 4-MET to calcium present in hydroxyapatite was positively demonstrated in the above mentioned study study using XPS which supported the proposed mechanism in which carboxylic group replaces the phosphate ions of the substrate and make ionic bonds with calcium ions of hydroxyapatite crystals.   1.3.1 Requirements for creating good adhesion  There are several sequential events that are required to form an effective adhesive joint. The key principles for good interface formation are creation of a clean surface, generation of a rough surface for interfacial interlocking, good wetting of the substratum by the adhesive materials, adequate flow and adaptation for intimate interaction, and acceptable curing when phase changes are required for final joint formation (Marshall et al., 2010).   12 1.3.1.1 Clean surfaces   A principal requirement for strong adhesive bonds is that the surface be clean and therefore in a high energy state. Films of water, organic debris, and / or biofilms are always present in the clinical situation, and interfere with wetting and spreading. Acid etching removes most of the contaminants, produces surface roughness for micro-mechanical interlocking, and forms facets on the mineral crystals (Baier, 1991). Etched enamel is wet readily by monomers, allowing good penetration, and forms micro-mechanical bonds easily.  1.3.1.2 Surface roughness  Wettability is enhanced for most practical dental situations by the presence of micro surface roughness (Wenzel, 1936) that states:  r = cos θ1/cos θ2 where r is the ratio of contact angles of the smooth and rough surfaces respectively. This equation predicts that for contact angles less than 90°, wetting is increased by surface roughness. but decreased for non - wetting materials with contact angle greater than 90°.The Wenzel effect has been confirmed for a number of dental material investigations including polymer surfaces (Busscher et al., 1984), cements (Milosevic, 1992) and composites (O'kane et al., 1993).   13 1.3.1.3 Proper contact angle and good wetting   For adhesion to occur the adhesive must wet the substratum as adhesion requires intimate contact of the materials to be joined. A prerequisite for adhesion is contact between the phases 1 and 2 forming the bond (Packham, 2003). The most common method of observing wetting is measuring the contact angle which is the internal angle in a droplet of liquid in contact with a solid. It represents the energetic equilibrium between the solid, liquid, and gas phases involved. Wetting is categorized from liquid (usually water, but not necessarily) contact angle as non-wetting ( > 90◦), wetting ( < 90◦), and spreading (∼0◦), although all liquids do wet all solids to some extent; degree of wetting equals degree of adhesion (Marshall et al., 2010) (Fig 1.1). The goal is always to select conditions that promote spreading, without going so low in liquid surface tension that the liquid’s cohesive strength is adversely diminished.   Figure 1.1 Contact angles indicating good wettability (left) and poor wettability (right )   14 1.3.1.4 Low viscosity adhesives and adequate flow   The adhesive generally must be low enough in viscosity and be capable of sufficient flow within the available application time to spread and adapt to the details of the adherent surface (Marshall et al., 2010).  1.3.1.5 Adhesive solidification  Enamel and dentin adhesive systems require polymerization of the liquid components as their final stage. Many of the dental situations requiring the use of adhesives are challenged by poor access for visible light curing. It is desirable for dental restorative resin to convert all of its monomer to polymer during polymerisation reaction as a requirement for creating good adhesion (Ferracane, 1985; Marshall et al., 2010).   Alternatives to acid etching   1.4.1 Air-abrasion  Another method of enamel pre-treatment, the air-abrasive technique (sandblasting) has been described in the literature and makes use of a high-speed stream of aluminum oxide particles (50-90 µm) propelled by air pressure  directly on the tooth surface creating surface roughness. Some researchers speculated that air-abrasion could make possible the direct bonding of orthodontic brackets without acid etching (Reisner et al., 1997).However other studies (Olsen et al., 1997) concluded that enamel surface preparation using air-abrasion results in a significant 15 lower bond strength and should not be advocated for routine clinical use as an enamel conditioner. In another study (Van Waveren Hogervorst et al., 2000) whose purpose was to quantify surface enamel loss and compare shear bond strength using acid etching in comparison to sand blasting technique showed that the bond strength of the sandblasted groups was significantly lower than that of the etching groups and concluded that sandblasting is not an alternative for the acid-etching technique currently used. Another study (Canay et al., 2000) has indicated that enamel surface preparation using sandblasting with a micro etcher alone results in a significantly lower bond strength and should not be advocated for clinical use as an enamel conditioner.  1.4.2 Laser etching  Laser etch has been reported to yield a surface suitable for bonding and also resistant to carious attack (Lee et al., 2003; Von Fraunhofer et al., 1993).While some studies comparing acid- or self-etching to laser etching have recommended the use of laser etching for orthodontic bonding, (Basaran et al., 2007; Özer et al., 2008) some have raised doubts over the usefulness of laser etching because of large standard deviations and coefficients of variation and the irregular surface topography caused by the procedure (Berk et al., 2008; Üşümez et al., 2002).  In the literature, there are conflicting reports about the use of lasers for enamel etching. Although some researchers have reported that the mean shear bond strength resulting from laser etching is lower than that from acid etching (Hossain et al., 2003; Lee et al., 2003; Martı́Nez-Insua et al., 2000) others have reported  favorable results with laser irradiation (Driessens, 1977; Walsh et al., 16 1994; Whitters et al., 2000).In another publication comparing sand blasting, acid etching and laser irradiated specimen the authors concluded that sandblasting and low-power laser irradiation (0.5, 0.75, and 1 W) are not capable of etching enamel suitable for orthodontic molar tube bonding, but 1.5 and 2.0 W laser irradiation may be an alternative to conventional acid etching (Özer et al., 2008).   Non-thermal plasma: as an alternative pre-conditioning procedure  Plasma which can be regarded as the fourth state of matter and is composed of highly excited atomic, molecular, ionic, and radical species. It is typically obtained when gases are excited into energetic states by radio frequency (rf), microwave, or electrons from a hot filament discharge. A plasma is a highly unusual and reactive chemical environment in which many plasma-surface reactions occur if it is directed towards a surface (Cheng et al., 2011). In particular, plasma modification of the surface energy of the materials can improve the adhesion strength, surface and coating properties, and biocompatibility (Boutonnet Kizling et al., 1996). Non-thermal plasmas combine exceptional chemical reactivity with a relatively mild, non-destructive character resulting from a cold gas phase (Yasuda, 2005).  Lehmann et al (Lehmann et al., 2013) found that plasma treatment lead to a significant increase of surface wetting with water and ethylene glycol for both surface (enamel and dentin) conditions. The reduction of the amount of carbon compounds on the surfaces which are transformed by chemical reactions is assumed to be the main reason for better wetting. Also the maximum surface temperature in the centers of the contact areas of the plasma jet with the tooth 17 substances during the scanning motion at the chosen scan velocity was found to be 32.6°C for enamel and no measurable modification of average roughness on enamel by plasma irradiation was found in this study. Studies (Chen et al., 2013) showed that without affecting the bulk properties, a super-hydrophilic surface could be easily achieved by the plasma brush treatment regardless of original hydrophilicity or hydrophobicity of dental substrates(enamel and dentin). Recently (Teixeira et al., 2014) demonstrated that NTP increased surface energy, surface wettability and bond strength between enamel and sealants potentially serving as a substitute for conventional acid etching procedures or as an adjuvant for self-etch sealants to enhance their bond strength.   Study rationale  Acid etching has been the gold standard in Orthodontics so far but has certain drawbacks like bond failures (Millett et al., 1998), technique sensitivity (Özer et al., 2008), higher incidence of white spot lesions (Knösel et al., 2012) and enamel loss during etching and de-bonding and clean up procedures (Hosein et al., 2004). Increased bond strengths are needed for premolars and molars (Zachrisson, 2007), fluorosed teeth (Lupan et al., 2011b), lingual retainers (Taner et al., 2012) which have increased failure rate. The effects of plasma treatment on enamel and dentin have not been completely understood yet as both these substances have different complex and inhomogeneous structures, which contain organic and inorganic components and need detailed investigations with respect to their responses to plasma treatment (Lehmann et al., 2013).Also research demonstrates that NTP increased surface energy, surface wettability and bond strength between enamel and sealants potentially serving as a substitute for conventional acid etching procedures or as an adjuvant for self-etch sealants (Teixeira et al., 2014) .This study also 18 suggested that improvements in bonding to enamel by NTP application could also benefit orthodontic bracket performance. With this background information it was considered worthwhile to explore the use of NTP for orthodontic bonding purpose especially so when very little research has been done in this area so far.   Therefore, the purpose of this in vitro study was to evaluate if NTP application after enamel acid etching could improve bracket to enamel bonding and also if NTP application by itself has a potential to bond brackets.    19  The research questions and null hypothesis were:     1) Could NTP application after enamel acid etching improve bracket to enamel bonding?             (Hₒ) =Shear bond strengths between brackets and enamel exposed to              NTP after acid etching will not be different from those achieved with acid etching alone.            (Hₐ) = Shear bond strengths between brackets and enamel exposed to              NTP after acid etching will be different from those achieved with acid etching alone.     2) Could NTP application by itself improve bracket enamel bonding?               (Hₒ) = Shear bond strengths between brackets and enamel exposed only                to NTP will not be different from those achieved with acid etching alone.               (Hₐ) = Shear bond strengths between brackets and enamel exposed only                to NTP will be different from those achieved with acid etching alone. 20 Chapter 2: Materials and methods   Materials used in the study 2.1.1 Bonding system Transbond™ XT adhesive paste (3M Unitek, Monrovia, CA), Orthosolo® primer and Ultra etch® etchant (Ultradent Products Inc.) were used for orthodontic bonding (Vicente et al., 2006; Vijayakumar et al., 2009). The Transbond™ adhesive paste (Image 2.1) is a composite resin and contains 10-20 % wt bisphenol A diglycidylether methacrylate (BIS-GMA), 5-10 % bisphenol A bis(2-hydroxyethyl ether) dimethacrylate (BIS-EMA),70-80 % wt silane treated quartz and less than 2 % silane treated silica. The Orthosolo™ primer (Image 2.2) is made predominantly of alkyl dimethacrylate resins and barium aluminoborosilicate glass (filler) .The Ultraetch® etching gel (Image2.3) is composed of 35 % phosphoric acid in water and amorphous silica.   2.1.2 Non-thermal plasma (NTP)  The plasma delivering equipment that was utilized in this study (KinPen™ 09, INP Greifswald, Germany) (Foest et al., 2005)—consists of a hand-held unit(170 mm length, 20 mm diameter, weighing 170 g) connected to a high-frequency power supply (frequency 1.1MHz, 2–6 kV peak-to-peak, 8 W system power) for the generation of a plasma jet at atmospheric pressure. The handheld unit has a pin-type electrode (1 mm diameter) surrounded by a 1.6 mm quartz capillary. An operating gas Argon at a flow rate of 5 slm (standard litres per minute) was used. The plasma plume emerging at the exit nozzle was about 1.5 mm in diameter and extended into the surrounding air for a distance of up to 15 mm (Image 2.7). 21   Image 2.1 Transbond™ XT adhesive (3M Unitek, Monrovia, California)  Image 2.2 Orthosolo® primer   22  Image 2.3 Ultraetch™ etching gel   Image 2.4 Orthodontic brackets: Elite® mini twin® (0.22 slot) Ortho Organizers Inc.   23  Image 2.5 Maxillary premolar bracket. Elite® mini twin® (0.22 slot) Cerum Ortho   Image 2.6 Light cure unit: Blue phase style, Ivoclar-Vivadent   24    Image 2.7 Non thermal plasma (KinPen™ 09, INP Greifswald, Germany) 25 Product Ingredients % by wt.  Transbond XT™  Silane Treated Quartz  Bisphenol A Diglycidyl Ether Dimethacrylate  Bisphenol A Bis(2-hydroxyethyl ether) Dimethacrylate  Silane Treated Silica  Diphenyliodonium Hexafluorophosphate  70-80  10-20  5-10  <2  <0.2 Orthosolo® Ethyl Alcohol  Alkyl Dimethacrylate Resins   Barium Aluminoborosilicate Glass   Fumed Silica (Silicon Dioxide)  Sodium Hexafluorosilicate  1-5%  60-80%  14-24%  2-10%  1-5% Ultra-etch® Phosphoric Acid   Cobalt aluminate blue spinel   Cobalt zinc aluminate blue spinel 32-38%  <1%  <1%  Table 2.1 Table of product chemical compositions    26  Experimental method  Prior to collecting results for the study, Research ethics board (REB) approval was obtained from the University of British Columbia to collect the teeth used in the study. No patient identifiers were associated with the teeth.  2.2.1 Teeth collection   Eighty-four maxillary first premolars were collected from two Maxillofacial and Oral surgery clinics and six dental clinics in Vancouver, BC over a six month period. The teeth were washed in water to remove any traces of blood and then placed in 0.5 % Chloramine T disinfectant at room temperature. They were then stored in distilled water which was changed periodically to avoid deterioration. These teeth were extracted for reasons other than the purposes of this study.  The criteria for tooth selection included characteristics like: intact buccal enamel, not subjected to any pre-treatment agents, no cracks and no caries on any surface.Exclusion criteria for the collected premolars included teeth with carious lesions, demineralisation, fluorosis, abfraction lesions, restorations, enamel defects, and visible evidence of abnormal cracking and attrition. Specimen teeth were excluded if any enamel damage was present including enamel craze lines or trauma from extraction forceps .    27  Sample size calculation  Calculations using mean bond strengths and standard deviations from a pilot study resulted in a minimum sample size of 7 per group in order to detect a 20 % difference in bond strength with 80 % power with 95 % confidence.   2.2.2 Groups  The extracted premolars were divided into two groups: no-treatment (control) and treatment group.The no–treatment (Control) group consisted of 12 premolars onto which orthodontic bracket bonding was performed without any enamel surface treatment. The treatment group consisted of 72 premolars which were further divided randomly into 3 main groups of 24 premolars each:  Group 1: Etch Group 2:  Etch + NTP Group 3:  NTP Each of the three treatment groups was subdivided into two subgroups of 12 premolars each according to the length of time of storage prior to testing, i.e. 24 hours or 1 month. The experimental design is summarized in Figure 2.1 below.   28    Figure 2.1 Control and treatment groups 29  Before bonding each enamel surface was cleansed and polished with residue free, non-fluoridated, non-flavoured pumice and water slurry for 10 seconds with a slow speed dental hand piece and rubber prophylactic cup. Each surface was then thoroughly rinsed with water and air-dried.  2.2.3 Bracket bonding procedure  Control group: Orthosolo® primer was applied in a thin film, lightly dried with oil-free air for 5 seconds, and cured for 10 seconds (Blue phase style @ 1,100 mW/cm², Ivoclar-Vivadent). (Image 2.6). Transbond XT™ adhesive paste was applied onto a metal orthodontic maxillary premolar bracket (Elite® mini twin® (0.22 slot) Ortho Organizers Inc.) ( Image 2.4 and 2.5), which was then placed on the enamel surface, adjusted to final position and pressed firmly to expel excess adhesive , which was removed with a scalar. Each side of the bracket was exposed to the light curing unit for 10 seconds for a total of 40 seconds, at a distance of no more than 1 mm from the bracket.  Experimental groups  Group 1: (Etch group): The buccal surface was etched with 37 % phosphoric acid ( Ultradent® etching gel) for 30 seconds, rinsed with water for 20 seconds, and dried with an oil-free air spray for 20 seconds until the enamel appeared frosty. Orthosolo® primer application and bracket placement was similar to as described for the Control group. 30 Group 2:(Etch +NTP group): After etching, rinsing and drying in the same way as described above for Group I the buccal enamel surface was exposed to NTP surface treatment. The hand held unit was kept perpendicular to the enamel at a distance of 2 mm and the surface was treated with NTP for 30 s. This was followed by bracket application as described for Control group. Group 3: (NTP group): The hand-held unit was kept perpendicular to the enamel at a distance of 2 mm and the surface was treated with plasma for 30 s. This was followed by application of the bracket similar to as described for Control group.  Ageing  The bonded tooth-bracket specimens were stored in deionized water in an incubator at 37 °C for (T1) 24 hours and (T2) 1 month prior to testing.  2.2.4 Specimen mounting   Guide for standardization of test specimen (GSTS): Ney’s Dental surveyor (A.M.D. Dental Manufacturing Inc.) was modified by attaching an assembly with a short segment of 0.021inch x0.025 inch (0.53mmx0.64mm) stainless steel wire (Image 2.8 and 2.9).  Specimen mounting: All teeth were mounted in PVC (polyvinylchloride) blocks before testing (Image 2.10). In order to achieve this, the teeth with bonded brackets were removed from the storage medium and attached to the GSTS orthodontic wire segment, using elastomeric ligatures.   31 PVC blocks (approximately 18-20 mm in height each) were prepared by manually cutting a full length PVC pipe (19 mm diameter). The diameter of the PVC blocks corresponded with the internal diameter of the adjustable vice (Image 2.13) and ensured a snug fit of the mounted specimen in the adjustable vice during testing. Then the vertical arm of the GSTS with the attached premolar was lowered so that the premolar’s root entered the PVC block approximately in its centre till the CEJ (cement-enamel junction) of the premolar approached 1mm higher than the upper edge of the block (Image 2.10). Caulk® Orthodontic Resin (Dentsply) acrylic was mixed according to the manufacturer’s recommendations and was poured into the PVC block and left undisturbed for 15 minutes, as per the manufacturer’s instructions (Image 2.11 and 2.12). The elastomeric mould was removed and the embedded premolar with its bracket was separated from the stainless steel wire of the GSTS. This procedure ensured reproducibility and proper orientation during testing.  32  Image 2.8 Ney’s dental surveyor  33  Image 2.9 (Guide for standardization of test specimen) -modified Ney’s surveyor  Image 2.10 Lowering premolar in the approximate center of the PVC mold 34  Image 2.11 Pouring acrylic after the premolar with bonded bracket is lowered in PVC block  Image 2.12 Final acrylic set   35 2.2.5 Testing procedure  After ten minutes, the mounted specimen were placed inside an adjustable vice for shear bond strength (SBS) testing in Shimadzu AGS-X Series Table-Top Type Precision Universal testing machine (Image 2.16) .The testing was accomplished by using a chisel edge mounted on crosshead of the testing machine. Each tooth was orientated such that the chisel (ODEME Company, Brazil ) (Image 2.15) was parallel to the bracket base and equidistant to both incisal tie-wings. The chisel-type working tip was positioned in the occluso-gingival direction in contact with the bracket-enamel junction, producing a shear force at the bracket-tooth interface until the bracket debonded .The speed of the cross head was set at 0.5 mm/min and the load was determined using 500 N load cell and recorded by the attached computer. The same de-bonding procedure was performed for all the samples. The force required to de-bond each bracket was recorded in Newtons (N) and SBS was calculated using the required de-bonding force and the measured bracket base surface area of 10.19 mm².After de-bonding, the enamel surface of each specimen was examined with 12x magnification to assess the amount of adhesive remaining on the tooth surface.   36  Image 2.13 Adjustable vice (Odeme company, Brazil)    Image 2.14 Chisel edge (Odeme company, Brazil)  37  Image 2.15 Orientation of blade to enamel- bracket interface  Image 2.16 Shimadzu AGS-X series table-top type precision universal testing machine   38 2.2.6 Adhesive remnant index  Once the brackets had been de-bonded, the enamel surface of each tooth was examined under ×12 magnification, with a stereomicroscope (Carl Zeiss Jena, Germany), (Image 2.17) to determine the amount of residual adhesive remaining on each tooth. The bond was qualitatively graded using Adhesive remnant index as described by (Årtun et al., 1984), with the following scale: 0 = no adhesive left on the tooth, 1 = less than half of the adhesive left on the tooth, 2 = more than half of the adhesive left on the tooth, and 3 = all adhesive left on the tooth, with a distinct impression of the bracket mesh. All procedures were performed by a single operator.    Image 2.17 Carl Zeiss Jena microscope, Germany    39 2.2.7 Scanning electron microscopy  Prior and after the different surface treatments described for the different groups, the characteristics of the surface were examined by scanning electron microscope (Hitachi S3000N, Japan) variable pressure Tungsten filament. It has a nominal resolution of 3.0 nm at 20 kV and was operated at an accelerating voltage of 15 kV in conventional high vacuum mode using the secondary electron detector. Samples were mounted on SEM stubs sputter-coated with gold.  One untreated intact premolar’s buccal surface was split into four equal parts using a diamond disc (K 6974, Komet, Lemgo, Germany) under water lubrication. Out of these 4 specimen, one was untreated control and the other three were subjected to respective three surface treatments of Etching (30 s), Etching (30 s) + NTP (30 s) and only NTP (30 s) and analysed under SEM at magnification of 500x, 1500x and 5000x.  Also one representative sample from each de-bonded sub-group was selected and its enamel surface and bracket base was examined at 30x magnification.     40  2.2.8 Statistical analysis  Separate one-way analyses of variance (ANOVA) were conducted to compare conditions on mean SBS scores at 24 hours (control and three treatments) and at 1 month (three treatments only). Post-hoc Tukey tests were used to evaluate differences at each level of treatment between groups. The non-parametric chi-square test statistical analysis was used to evaluate the difference between adhesive remnant index score of different surface treatments at 24 hours and 1 month. The statistical analysis of the results was conducted using SPSS program (SPSS, Chicago, Ill) and significance was established at α= 0.05. 41 Chapter 3: Results   SBS analysis  Table 3.1 displays descriptive statistics for SBS (in MPa) for the control group and three experimental groups (n = 12) at 24 hours, and for the three experimental groups (n = 12) at 1 month. Time of assessment Surface treatment N Mean SD SEM Min Max 24 Hours Control 12 2.4 0.85 0.24 1.09 3.94  NTP 12 17.5 2.96 0.85 14.27 23.00  Etching 12 22.9 6.58 1.90 11.04 32.79   Etching + NTP 12 24.4 4.40 1.27 16.03 31.24 1 Month NTP 12 11.9 3.65 1.05 6.21 19.06  Etching 12 18.5 5.78 1.67 10.27 28.15   Etching + NTP 12 12.8 4.52 1.31 3.67 19.50 Table 3.1 Descriptive statistics for SBS (MPa).    42  Box-and-whisker charts were generated for each condition at each time of assessment (Fig 3.1). Although some skew was observed in most treatment distributions, in general medians were near the middle of the interquartile range and whiskers of the upper and lower quartiles were similar in length. However, the box-plot for the control sample indicated a substantially narrower distribution of scores.  Figure 3.1 Box-and-whisker charts of SBS at 24 hours and 1 month   43 Separate one-way analyses of variance (ANOVA) were conducted to compare conditions on mean SBS scores at 24 hours (control and three treatments) and at 1 month (three treatments only). Post-hoc Tukey tests were used to evaluate differences at each level of treatment between groups.  3.1.1 Comparisons at the 24hour follow-up  A One-way between subjects ANOVA was conducted to compare the control (no surface treatment on enamel) and the effect of 3 surface treatments on shear bond strength 1) when enamel was being treated by NTP application only, 2) when enamel was treated with etching first followed by NTP treatment and 3) when enamel was treated with etching only after 24 hours of ageing in distilled water at 37 °C. Normality test passed using Shapiro- Wilk test and Equality of variance failed using (Levene’s test). Power of the performed test with alpha=0.05:1.000. The one-way ANOVA on data at 24 hours revealed a significant effect for treatment, F (3, 44) = 67.12 (see Table 3.2). Although the Levene’s statistic indicated that the homogeneity of variance assumption had been violated (FL [3, 44] = 6.39, P < .05), the robust Welch’s F* statistic still found a significant effect for treatment, F*(3, 20) = 200.97, P < .05.   44 Source SS df MS F P Treatment 3627.93 3 1209.31 67.12 < .05 Error 792.72 44 18.02   Total 4420.65 47    Table 3.2 One-way analysis of variance (ANOVA) on SBS at 24 hours  Post -hoc adjusted comparisons using the Tukey HSD test (Table 3.3) indicated that SBS at 24 hours was greater in all three treatments than in the control group. Surface treatment Mean difference in bond strengths (MPa) P -value E+NTP vs. C 21.9 < 0.001* E+NTP vs. NTP 6.9 0.001* E+NTP vs. E 1.5 0.822 E vs. C 20.4 < 0.001* E vs. NTP 5.4 0.016* NTP vs. C 15.0 < 0.001* Table 3.3 Pairwise multiple comparison procedures (Tukey Test) A significant difference in the shear bond strengths when all 3 surface treatments, NTP (Mean = 17.4, SD = 2.9), Etch (Mean = 22.8, SD = 6.5) and Etch +NTP (Mean =24.3, SD= 4.3) were compared to Control (Mean = 2.3, SD = 0.8). Mean SBS for NTP treated surfaces (Mean = 17.4, SD = 2.9) was significantly lower than the Etch (Mean = 22.8, SD = 6.5) and Etch +NTP (Mean= 24.3, SD= 4.3) treated surfaces. However, the Etch treatment did not significantly differ from the Etch +NTP treatment (P =.0.82). In summary at 24 hours the ordinality of SBS was: control < NTP < Etching =Etching+ NTP  45 3.1.2 Comparisons at the 1 month follow-up  A one-way between subjects ANOVA was conducted to compare the effect of 3 surface treatments on SBS under three surface conditions: 1) when enamel was being treated by NTP application only, 2) when enamel was treated with Etching first followed by NTP treatment and 3) when enamel surface treated with Etching only, after 1 month of ageing in distilled water at 37°C. Normality test passed using Shapiro - Wilk test and Equality of variance passed using (Levene’s test). Power of the performed test with alpha = 0.05:0.862. The one-way ANOVA on data at 1 month revealed a significant difference in shear bond strength at the P < .05 level for the three conditions F (2, 33) = 7.02 (Table 3.4).This time the Levene’s statistic found that the homogeneity of variance assumption had not been violated (FL [2, 33] = 2.30, P = .12). Source SS df MS F P Treatment 314.31 2 157.15 7.02 < .05 Error 739.11 33 22.40   Total 1053.41 35     Table 3.4 One-way analysis of variance (ANOVA) on SBS at 1 month   46 Post -hoc adjusted comparisons using the Tukey HSD test (Table 3.5) indicated that the SBS for Etch treated surfaces (Mean = 18.5, SD = 5.7) was significantly higher than the NTP (Mean = 11.8, SD = 3.6) and Etch +NTP  treated surface (Mean = 12.7, SD = 4.5) .  However, the Etch +NTP did not significantly differ from the NTP treatment (P=0.88). In terms of ordinality of SBS at 1 month, NTP= Etching + NTP < Etching. Surface Treatment Mean difference in bond strengths (MPa) P -value E vs. NTP 6.6 0.004* E vs. E +NTP 5.7 0.014* E+NTP vs. NTP 0.8 0.889 Table 3.5 Pairwise multiple comparison procedures (Tukey Test)  3.1.3 Two-way ANOVA (surface treatment and ageing time interaction) A two-way ANOVA which was employed to examine the effects of two factors: 1) three surface treatments (NTP, Etch +NTP and Etch) and two time periods (at 24 hours and 1 month) on SBS. Only the Treatment x Time interaction term was evaluated in the customized model. Results of the two-way ANOVA are presented in (Table 3.6). The interaction was confirmed, F (5, 66) = 13.49, and the homogeneity of variance assumption had not been violated (FL [5, 66] = 1.89, P = .11).   47   Source SS df MS F P Interaction 1557.59 5 311.52 13.49 < .05 Error 1523.91 66 23.09   Total 3081.50 71    Table 3.6 Customized two-way analysis of variance (ANOVA) on SBS x Time  To determine the effect of ageing time (24 hours vs. 1 month) within each surface treatment group (Etch, Etch +NTP and NTP) a pairwise multiple comparison Post-hoc Tukey test was done (Table 3.7).  Mean difference in bond strengths (MPa) P- value Time within NTP  5.58 0.006* Time within Etch+ NTP 11.63 0.031* Time within Etch 4.32 < 0.001* Table 3.7 Pairwise multiple comparison (Tukey Test)  Post -hoc adjusted comparisons using the Tukey HSD test indicated that all 3 surface treatments (Etch, Etch +NTP and NTP) showed significant difference in SBS when intragroup comparisons were done for ageing time (24 hours vs.1 month ) (P < 0.05). Etch +NTP showed maximum mean difference of 11.63 Mpa (drop in SBS over time) and Etch group showed minimum difference of 4.32 MPa .  48  Figure 3.2 Line chart of decline SBS from 24 hours to 1 month  Comparisons of decline in SBS from 24 hours to 1 month in the three treatment groups  (Figure 3.2) suggested that surface treatment interacted with ageing time. Specifically, although SBS declined from 24 hours to 1 month in all three samples, the rate of decline appeared to be higher in the Etching + NTP and the NTP group but not in the Etching group.  Therefore the null hypothesis that the shear bond strengths between brackets and enamel exposed only to NTP will not be different from those achieved with acid etching alone is rejected. Also the other null hypothesis that the shear bond strengths between brackets and enamel exposed to 49 NTP after acid etching will not be different from those achieved with acid etching alone was also rejected.   Adhesive remnant index analysis  After de-bonding, the teeth and brackets were examined under 12x magnification to evaluate the amount of resin remaining on the tooth. The adhesive remnant index (ARI) (Årtun et al., 1984) was used to describe the quantity of resin remaining on the tooth surfaces. The ARI score has a range between 0 and 3 as follows: 0, no adhesive remained on the tooth; 1, less than half of the enamel bonding site was covered with adhesive; 2, more than half of the enamel bonding site was covered with adhesive; and 3, the enamel bonding site was covered entirely with adhesive.     ARI Scores   Group n 0 1 2 3 Nt 12 12    - - - NTP 12 11 1 - - Etch+ NTP 12 - - 5 7 Etch 12 - 1 1 10 Table 3.8 Frequency distribution of the (ARI) , of experimental groups-24 hours  Table 3.8 shows frequency distribution of ARI scores of the control and 3 experimental groups after 24 hours of ageing. The results of chi-square test showed that there was a statistically significant difference in the ARI values among the treatments groups (Χ², (9, 48) = 54.5, P = <0.05). Pair-wise multiple comparisons indicated significant differences amongst Etch +NTP vs. 50 Nt , Etch +NTP vs. NTP, Etch vs. Nt and Etch vs. NTP (P  <0.05). No statistical difference was found between Etch+ NTP vs. Etch (P = 0.12) and Nt vs. NTP (P = 0.31).     ARI Scores   Group n 0 1 2 3 NTP 12 9 3 - - Etch+NTP 12 2 0 10 0 Etch 12 0 1 10 1 Table 3.9 Frequency distribution of the (ARI) of experimental groups-1 month  Table 3.9 shows frequency distribution of ARI scores of 3 experimental groups after 1 month of ageing. The results of chi-square test showed that there was a statistically significant difference in the ARI values among the treatments groups (X² (9, 48) = 45.8, P = < 0.05). Pair-wise multiple comparisons indicated significant differences between Etch+ NTP vs. NTP and Etch vs. NTP (P < 0.05). No statistical difference was found between Etch+ NTP vs Etch (P = 0.70).   51  SEM evaluation  3.3.1 SEM evaluation of enamel surfaces  Images (3.1) to (3.12) show the enamel surface under SEM (500x, 1500x and 5000x) after No- treatment (Nt-control), Etching (Group 1), Etch + NTP application (Group 2), and only NTP application (Group 3) respectively.   Group 3 (Images. 3.10 - 3.12) showed no changes on surface morphology and was similar to Nt-control group (Images. 3.1- 3.3) which showed some pores and some areas which comprised superficial irregularities such as grooves.  (Group 1) (Images.3.4- 3.6) and (Group 2) (Images.3.7-3.9) had similar etching patterns among them (Type 1) and enamel surface topography showed that enamel prisms were hollowed out to deep pits or craters placed side by side separated by thick inter-prismatic enamel persisting in the form of rings. They had a rough and uneven surface which indicated alteration of prismatic structure of the enamel due to selective dissolution of apatite crystals. The loss of superficial structure was evident.  52   Image 3.1 SEM (500x) photo of (Nt-control) enamel surface  Some pores and superficial irregularities such as grooves are seen. 53   Image 3.2 SEM (1500x) photo of (Nt-control) enamel surface Some pores and superficial irregularities such as grooves are seen.  54  Image 3.3 SEM (5000x) photo of (Nt-control) enamel surface Some pores and superficial irregularities such as grooves are seen.  55  Image 3.4 SEM (500x) photo of enamel surface exposed to acid etching (30 s). The enamel showed surface irregularities typical of a Type I etching pattern.  56  Image 3.5 SEM (1500x) photo of enamel surface exposed to acid etching (30 s). The enamel showed surface irregularities and etching of prism cores typical of a Type I etching pattern.  57  Image 3.6 SEM (5000x) photo of enamel surface exposed to acid etching (30 s). The enamel showed surface irregularities typical of a Type I etching pattern.  58  Image 3.7  SEM (500x) photo of enamel surface exposed to acid etching (30 s) + NTP (30 s)  Type I acid etched pattern is seen. The enamel showed surface irregularities typical of a Type I enamel etching pattern; etching of prism cores was predominant.  59  Image 3.8 SEM (1500x).photo of enamel surface exposed to acid etching (30 s) + NTP (30 s) Type I acid etched pattern is seen. The enamel showed surface irregularities typical of a Type I enamel etching pattern; etching of prism cores was predominant.  60  Image 3.9 SEM (5000x) photo of enamel surface exposed to acid etching (30 s) +NTP (30 s) Type I acid etched pattern is seen. The enamel showed surface irregularities typical of a Type I enamel etching pattern; etching of prism cores was predominant.   61   Image 3.10 SEM (500x) photo of enamel surface exposed to NTP (30 s) . No changes on surface morphology are seen and enamel surface is similar to as that of control (Nt- group). 62   Image 3.11 SEM (1500x) photo of enamel surface exposed to NTP for (30 s)  No changes on surface morphology are seen and enamel surface is similar to as that of control (Nt- group).   63  Image 3.12 SEM (5000x) photo of enamel surface exposed to NTP (30 s)  No changes on surface morphology are seen and enamel surface is similar to as that of control (Nt- group).   64 3.3.2 SEM evaluation (adhesive remnant index)  To determine the amount of resin remaining on the surface of the teeth at T1 (24 hours) and  T2 (1 month) microphotographs of the enamel surface were taken. A representative sample from each sub-group was selected and its bonded enamel surface and bracket base was examined at 30x magnification of SEM and was assessed with the adhesive remnant index described by Artun and Bergland (Årtun et al., 1984).  Under SEM (30x), samples in the untreated control groups and NTP (24 hours and 1 month ) group  showed a majority of adhesive failure and presented with a smooth and almost clean enamel surfaces after debond, often reflecting original perikymata. ARI score of 0 was calculated in these sub-groups. The bracket base could not be outlined in-spite of the fact that extra care was taken to remove excessive flash during bonding for all specimen (Image 3.13 ,3.16 and 3.19).This excessive adhesive was paper thin in consistency and presumably is the polymerized primer which also got detached during de-bond and obscured the bracket outline.  Etch (24 hours) and Etch + NTP (24 hours) (Image 3.14 and 3.15) showed a greater number of cohesive failures with almost all adhesive remnants were seen on enamel surface (minimal adhesive seen on the bracket base) indicating good adhesion of adhesive to enamel. ARI score of 3 was calculated in these sub-groups Also a well-defined bracket mesh and bracket outline was seen.  65 A majority of samples in the 1 month (Etch and Etch + NTP) groups had a mix of cohesive and adhesive failures, with an ARI score of 2.  SEM (30x) showing a mix of cohesive and adhesive failure with about 75 % - 80 % adhesive remnants are seen on enamel surface and about  20 - 25 % at the bracket base (Image 3.17 and 3.18).   66                  Bonded enamel surface                                                                         Bracket base    Image 3.13 SEM (30x) photo of a representative sample from no-treatment (control) group (24 hours)  No adhesive remnant was seen on the enamel surface after de-bonding as most of the bonding material is seen on bracket base. Maximum number of bond failures occurred at the enamel-adhesive interface resulting in an ARI score of 0. The majority of adhesive is attached to the bracket base with nearly 100 % of the bonded enamel surface free of any adhesive remnant indicating a stronger adhesion to metal bracket base than to enamel.   67                    Bonded enamel surface                                                                             Bracket base     Image 3.14 SEM (30x) photo of a representative sample from Etch (24 hours) group  Cohesive failure with almost all adhesive remnants are seen on enamel surface (minimal adhesive seen on the bracket base) indicating good adhesion to enamel. After de-bonding the predominant mode of failure was bracket-adhesive interface and corresponding to ARI score of 3.   68                             Bonded enamel surface                                                         Bracket base    Image 3.15 SEM (30x) photo of a representative sample from Etch +NTP (24 hours) group  Cohesive failure with almost all adhesive remnants are seen on enamel surface (minimal adhesive seen on the bracket base) indicating good adhesion to enamel and a corresponding ARI score of 3.  After de-bonding the predominant mode of failure was at bracket-adhesive interface.   69                      Bonded enamel surface                                                          Bracket base    Image 3.16 SEM (30x) photo of a representative sample from NTP (24 hours) group  No or minimal adhesive remnants on enamel surface after de-bonding are seen. Maximum bond failures occurred at the enamel-adhesive interface resulting in an ARI score of 0.    70                       Bonded enamel surface                                                  Bracket base    Image 3.17 SEM (30x) photo of a representative sample from Etch (1 month) group  Adhesive remnants were seen on both the enamel surface and at bracket base. A mix of cohesive and adhesive failure is seen with about 75 %-80 % adhesive remnants are seen on enamel surface and about 20-25 % at the bracket base. After de-bonding the predominant mode of failure was bracket-adhesive interface and a corresponding ARI score of 2.   71                                Bonded enamel surface                                          Bracket base  Image 3.18 SEM (30x) photo of a representative sample from Etch+ NTP (1 month) group  After de-bonding the predominant mode of failure was bracket-adhesive interface resulting in an ARI score of 2. Adhesive remnants were seen on both the enamel surface and at bracket base. A mix of cohesive and adhesive failure is seen.    72                                   Bonded enamel surface                                                       Bracket base    Image 3.19 SEM (30x) photo of a representative sample from NTP (1 month) group  No adhesive remnant seen on enamel surface after de-bonding. Maximum bond failures occurred at the enamel-adhesive interface resulting in an ARI score of 0.   73 Chapter 4: Discussion   Phosphoric acid etching is the gold standard method of enamel pre-conditioning for orthodontic brackets bonding with resin based system (Driessens, 1977). However this technique is fraught with certain limitations including bond failures (Millett et al., 1998) (O'brien et al., 1989), technique sensitivity (Berk et al., 2008), higher incidence of white spot lesions (Knösel et al., 2012), enamel loss during etching (Legler et al., 1990), difficult de-bonding and clean up procedures (Hosein et al., 2004) and retention of resin tags that can lead to subsequent potential discolouration of enamel (Gwinnett, 1971; Silverstone, 1977). Ideally, high bond strength between non-etched enamel and orthodontic adhesive is desirable, a challenging scenario not only due to the lack of mechanical interlocking between enamel and resin but also due to limited surface area relative to the etched counterpart (Miyazaki et al., 2003). Non-thermal plasma (NTP) has been used recently to modify enamel (Lehmann et al., 2013) and dentin (Ritts et al., 2010) surfaces and also improve the interfacial bonding of dental composite restorations to dentin (Dong et al., 2013) and enamel (Teixeira et al., 2014) surfaces.   The present study evaluated if NTP application by itself has a potential to bond orthodontic brackets, and if the effect of NTP after enamel acid etching could improve bracket to enamel bonding.  Our results showed that SBS for NTP group at (17.5 MPa) was lower than for the Etching group (22.5 MPa) after 24 hours. The same trends continued after 1 month of ageing, with the SBS in NTP group at 11.9 MPa and in Etch group at 18.9 MPa. SBS of brackets bonded after acid 74 etching and using Transbond™ XT adhesive (3M Unitek, Monrovia, California) tested after 24 hours of ageing was 22.5 MPa in this study. This result is similar to that reported by other studies which evaluated the SBS of orthodontic brackets using Transbond XT™ light-cure composite after phosphoric acid etching (35 %) (Montasser et al., 2009; Oesterle et al., 2008; Rix et al., 2001).  Image 4.1 SEM micrograph (5000x) of enamel surface exposed to acid etching for 30 s showing Type 1 etching pattern and increased surface roughness Van Meerbeek and his group (Van Meerbeek et al., 2002) have shown using SEM that the mechanism of resin adhesion in enamel bonding after acid etching involves the formation of “macro” and “micro” resin tags within etch pits at the cores of etched enamel prisms. SEM images in our study confirmed that a Type 1 etching pattern was present in the Etch group (Image 4.1) and this is in agreement with other studies that noticed similar etch pattern (Legler et 75 al., 1989). This surface roughness would have accounted for the increased SBS values obtained for the Etched group, due to increased mechanical interlocking of resin tags between the enamel substrate and the resin. NTP treated specimen demonstrated SBS of 17.5 MPa after 24 hours of ageing. There are no studies reported in literature which have tested SBS values after NTP surface treatment or compared SBS between Etch and NTP treated and bonded orthodontic specimen.  Image 4.2 SEM micrograph (5000x) of enamel surface exposed to NTP for 30 s. No surface morphological changes are seen   76 Although SEM characterization, both in this study (Image 4.2) and other studies (Chen et al., 2013; Teixeira et al., 2014) showed no surface changes after NTP surface treatment, the SBS after NTP treatment was still significantly higher than that of untreated Control group (2.4 MPa). This is in agreement with (Teixeira et al., 2014), who has shown that NTP surface treatment increased surface energy, surface wettability and bond strength between enamel and sealants and concluded that the increase in polarity for NTP group accounted for the increased µ-SBS values when compared to untreated controls. Other studies (Chen et al., 2013; Lehmann et al., 2013) have also demonstrated the effect of NTP on enamel surface, including reduction of contact angle and carbon percentage on enamel and concluded that a super-hydrophilic surface could easily be achieved by the plasma brush. Chu and co-workers (Chu et al., 2002) observed that NTP presents potential for enamel bonding improvements as chemical radicals may be deliberately incorporated in the non-etched enamel. All these factors could have accounted for increased SBS in the NTP group when compared to untreated controls. However, NTP group resulted in lower SBS when compared to Etching group due to lack of etching and, consequently, lack of micromechanical tags.  This is supported by Marshall and his co-workers (Marshall et al., 2010) who noted that bonding involves potential contributions from physical, chemical and mechanical sources but primarily relies on micromechanical interaction for success.  Both NTP and Etch group showed a decline in SBS after 1 month of ageing when compared to 24 hours. For the Etch group, SBS dropped from 22.5 MPa to 18.9 MPa (16 %), a drop that may be attributed to ageing of composite in water environment and its subsequent weakening (Oesterle et al., 2008). 77 This finding is in accordance with other studies (De Munck et al., 2004; Drummond, 2008; Koin et al., 2008; Oesterle et al., 2008) that reported similar decrease in SBS after ageing of bonded orthodontic specimen in water. In the NTP group, SBS dropped from 17.5 MPa to 11.9 MPa  (32 %), a significantly higher drop than the one recorded for the Etched group. Water sorption might also have lead to decreased SBS after ageing of NTP treated specimen and lack of deep mechanical interlocking as seen in Etch counterpart may have attributed to a greater drop. However there are no studies reported on ageing of NTP treated orthodontic brackets so far.   NTP surface treatment after acid etching did not have any significant enhancing effect on SBS in Etch+ NTP group after 24 hours and seemed to have a detrimental effect on SBS when bond strengths were tested after 1 month of ageing when compared to Etch group. Studies (Teixeira et al., 2014) have shown increased SBS in Etch + NTP group when NTP was used to modify etched and non-etched enamel surfaces to enhance physicochemical interaction with dental sealants to enamel. A similar outcome was expected with regards to bracket bonding, considering that the etched enamel surface subjected to NTP would have an increased surface hydrophilicity, which in-turn would improve the penetration of the dental adhesive  for better micromechanical interlocking and increased SBS. However, no significant differences in SBS between Etch +NTP and Etch group were identified, even if the SBS of the Etch + NTP group was the highest  (24.4 MPa) after 24 hours of ageing. One explanation for this could be that resin penetration was probably equally deep in the Etch + NTP and in the Etch group and that the increase in hydrophilicity after NTP surface treatment as shown by various studies (Lehmann et al., 2013) (Chen et al., 2013) did not play a significant role in enhancing SBS for Etch +NTP group.   78 Interestingly, after 1 month of ageing, the SBS dropped significantly in Etch+ NTP group, from 24.4 MPa to 12.8 MPa (47 %), a much bigger drop than the one recorded for the Etch group, which dropped from 22.5 MPa to 18.9 MPa (16 %). The overall generalized decrease in SBS in both groups can be attributed to ageing (Drummond, 2008; Ferracane et al., 1998) and to the effect of water sorption that might have plasticized the polymers and lowered their mechanical properties (Bastioli et al., 1990) . A greater decline in the  SBS of the Etch+NTP (1 month) group may be explained by the increased hydrophilicity of the enamel after NTP surface treatment, leading to an increase in susceptibility to exposure to water, similar to what has been shown to happen to self-etching adhesives. Pashley and co-workers (Tay et al., 2003) have shown that one of the main disadvantages of one-step self-etch adhesives is related to their excessive hydrophilicity that makes the adhesive layer more prone to attract water. As these adhesives are more susceptible to water sorption and thus nano-leakage, they are also more prone to bond degradation and tend to fail prematurely as compared to their multi-step counterparts (Cardoso et al., 2011). It is known from the literature (Chen et al., 2013) that NTP surface treatment makes the enamel surface super-hydrophilic, which might have resulted in a similar process of attracting more water during ageing, leading to greater water sorption and bond degradation, resulting in a significant drop in SBS and lower SBS values when compared to the Etch group.  SBS should be within an optimum range between 5.8 MPa-13.5 MPa to be supposedly “clinically acceptable” as recommended by Rossouw (Rossouw, 2010) with about 10 MPa as mean value. It was interesting to note that no bracket in all three treatment groups (24 hours) and Etch (1 month) failed at SBS below 10 MPa. However 3 out of 12 brackets (25 %) in Etch+ NTP 79 (1 month) and 4 out of 12 brackets (33 %) in NTP (1 month) failed at an SBS below this value which may raise some concerns about the use of NTP for bonding.  Bracket failure at either of the two interfaces, bracket-adhesive interface or enamel-adhesive interface, has its own advantages and disadvantages (Bishara et al., 2007). Failure at the bracket-adhesive interface is advantageous as it indicates good adhesion to the enamel and is safer to de-bond (Berk et al., 2008) . However, considerable chair time (Khoroushi et al., 2007) is needed to remove the residual adhesive, with the added possibility of damaging the enamel surface during the cleaning process (Justus et al., 2010) . Also more enamel loss during cleaning is reported (Bishara et al., 2000). In contrast, when failure occurs at the enamel-adhesive interface, less residual adhesive remains on the enamel and less chair-side time is needed. for cleaning however failure at this interface may cause enamel fracture while de-bonding (Berk et al., 2008) .  In our study, the majority of the brackets bonded in the NTP group and in untreated control group had adhesive failures at the enamel-adhesive interface, whereas brackets bonded using Etch, Etch+ NTP had more cohesive/mixed failures and failed more often at the bracket-adhesive interface. The difference in fracture sites is due to difference in effects of surface treatments and is supported by our SEM analysis which showed Type 1 patterns with Etch and Etch+ NTP group. This is in agreement with other studies (Bonetti et al., 2011; Özer et al., 2008; Romano et al., 2006) which also showed cohesive/mixed failures for the Etch treated specimen. Also (Teixeira et al., 2014) noted cohesive/mixed fracture sites for the Etch+ NTP group and our results are similar to their findings. NTP surface treatment does not appear to cause any surface modification of enamel surface, as was also seen in other studies (Chen et al., 2013; Teixeira et 80 al., 2014). The absence of any surface demineralization might explain the failure site with minimal adhesive remnants on the enamel surface, as was also seen in the untreated controls. Our findings are similar to (Teixeira et al., 2014) who observed that the NTP treated group and Control group had all adhesive fractures.  Under SEM (30x), samples in the NTP 24 hours and 1 month groups and untreated control groups presented smooth and almost clean enamel surfaces after de-bond, often reflecting original perikymata (Image 4.3) and an ARI score of 0.                                Enamel surface                                                               Bracket base       Image 4.3 SEM (30X) NTP (1 month)   SEM (30x) showing adhesive failure with almost clean enamel surface and obscure bracket base outline. The majority of adhesive is attached to the bracket base with nearly 100 % of the bonded enamel surface free of any adhesive remnant indicating a stronger adhesion to metal bracket base than to enamel .    81 On the contrary, samples in the 24 hours Etch and Etch+ NTP groups (Image 4.4) showed a well-defined bracket mesh and bracket outline. A majority of samples in these two groups had cohesive failures with an ARI score of 3.      Image 4.4 SEM (30X) Etch (24 hours)   SEM (30x) showing cohesive failure with almost all adhesive remnants are seen on enamel surface (minimal adhesive seen on the bracket base) indicating good adhesion to enamel.  Also the majority of samples in the 1 month Etch and Etch+ NTP groups had a mix of cohesive and adhesive failures, with an ARI score of 2 (Image 4.5) .   82    Image 4.5 SEM (30 X) Etch (1 month)   SEM (30x) showing a mix of cohesive and adhesive failure with about 75 %-80 % adhesive remnants are seen on enamel surface and about 20-25 % at the bracket base with ARI score of 2 .   There has also been debate whether or not ARI scores reflect a difference in bond strength (Montasser et al., 2009). In our study, some trends of relationship between SBS and ARI were noted in Etch and Etch+ NTP group. When SBS was higher, as in the Etch (24 hours) (22 MPa) and Etch +NTP (24 hours) (22 MPa), the ARI score was 3 and as SBS dropped to lower values after 1 month of ageing, to 18 MPa and 12 MPa, respectively, the ARI score dropped to 2. This observation is supported by other studies who reported similar relationship between ARI and SBS (Parrish et al., 2011) (Mirzakouchaki et al., 2012).However, caution must be taken when comparing SBS with ARI scores. Comparable SBS can be achieved in samples of two different groups even if the failures take place at different locations resulting in different ARI scores. In this study, for example, comparable SBS values showed different ARI scores. SBS of approximate 18 MPa was recorded for both NTP (24 hours) and Etch (1 month) groups, though their respective ARI scores were 0 and 2.  83 Also, approximate12 MPa was recorded in the NTP (1 month) and Etch+ NTP (1 month ) groups, though their respective ARI scores were 0 and 2.This observation is supported by other studies which have shown that the amount of adhesive remnant on the enamel surface following de-bonding is not related to SBS, but is instead governed by numerous factors, including bracket base design and adhesive properties (Dorminey et al., 2003; O'brien et al., 1988) ;(Endo et al., 2008) (Romano et al., 2006);(Zeppieri et al., 2003).  84 Chapter 5: Conclusions  NTP treated specimen had SBS values higher than the recommended values of 5.9-7.8 MPa (Reynolds, 1975), close to those recommended by Bishara et al (Bishara et al., 1993) of  11.5 MPa, and within an optimum range between of 5.88 MPa and 13.53 MPa  as recommended by Rossouw (Rossouw, 2010). Such unprecedented improvement in bond strength to non- etched substrates demonstrates the effectiveness of NTP treatment in increasing the surface energy and bonding ability of the system without compromising the surface morphology and substrate mechanical properties, highly desirable features for enamel bonding (Teixeira et al., 2014). It would be interesting to see how NTP would behave with increased ageing, over a typical orthodontic treatment time that averages 23.3 months to 33.4 months (Beckwith et al.) or 13 months to 40 months (Stewart et al., 2001).  The relatively short ageing time used in this study is a limitation which was due to time constraints. 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